UC Master Gardeners

Leaves of blue oak trees brown early due to drought

Special to The TribuneJuly 23, 2013 

Q. My blue oak tree and many others in the oak woodland are browning and shedding their leaves. What is the cause of this and how can I prevent it?

A. The blue oak is a California native tree that covers the western coast of North America. Named for the bluish tint of its leaves, most apparent during spring flush, it is a sturdy tree capable of withstanding high temperatures and long periods of drought.

In appearance, it is generally a medium-size tree with an open canopy. It grows to about 30 feet; however, when provided deep, moist soil, a height of 60 feet is possible. From the beginning, the blue oak spreads deep roots that allow it to survive dry conditions. The extensive root system can tap into water sources through cracks in rocks that are 80 feet down. This strength allows the tree to survive fire and regenerate from burnt or cut stumps.

The blue oak is a deciduous tree that drops its leaves on a predictable schedule. Nights get cool, days get short. Suddenly, November arrives and it's time for the blue oak to prepare for winter and go dormant. Leaves fall.

Some years, however, things just don't go as planned!

During the late 1970s and late 1980s severe drought conditions plagued California. At that time, many blue oaks began dropping their leaves in August―or about three months early.

As tough as the blue oak is, it still requires up to 300 gallons of water daily. When water supplies become depleted, to conserve water, leaves turn brown and drop to the ground, much as they ordinarily do in late autumn. The tree, in an attempt to defend itself against an unfriendly environment, goes dormant.

This is happening even earlier this year than in previous drought years. Of course, it is also unusually dry this year. Trees with this mechanism are often coined, "drought deciduous."

This is a naturally occurring cycle of the Blue Oak. It would have shed its leaves anyway, it is just doing it earlier due to the lack of rainfall this past winter.

The tree is not expecting water at this time of year so if you did water, you could potentially cause root rot and other problems. The tree will not die from this defoliation and will be fine until the rains return.

Got a gardening question?

Contact the University of California Cooperative Extension Master Gardeners at 781-5939 from 1 to 5 p.m. on Monday and Thursday; at 473-7190 from 10 a.m. to noon on Wednesday in Arroyo Grande; and at 434-4105 from 9 a.m. to noon on Wednesday in Templeton. Visit the UCCE Master Gardeners website at http://ucanr.edu/sites/mgslo/ or e-mail mgsanluisobispo@ucdavis.edu.

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